This invention relates to electronic speed governors for internal combustion engines. More particularly, this invention relates to such speed governors that use an electromagnetic actuator responsive to a drive control signal.
Several types of electronic governors are well-known in the art. In a typical electronic governor, a control signal is output by a microprocessor to an electromagnetic actuator, which in turn is interconnected with the engine throttle. The control signal is pulse width modulated, with the duty cycle being a function of the desired throttle opening. The actuator force applied to the throttle is opposed by a spring force of a return spring, which tends to bias the throttle closed at larger throttle openings. The actuator force is also opposed by frictional forces of the system, and other forces resulting from the off-center arrangement of the throttle valve. An off-center throttle also tends to bias the throttle to the closed position at larger throttle openings. The control signal typically has a fixed frequency.
A problem with the prior art electronic governor occurs when the inductance in the actuator become sufficiently large to create a 50 percent or greater duty cycle. This corresponds to approximately fifty percent throttle valve opening. At this level, the actuator inductance becomes sufficiently large so that it is difficult to make relatively small adjustments in the throttle plate opening and thus in engine speed. Small adjustments are desirable so that the operator does not audibly notice changes in the engine speed. These small changes are on the order of 10 to 15 rpm.
When the inductance reaches this level, and at a control signal frequency of 60 hertz or greater, the OFF time of the control signal is not long enough to enable the actuator's magnetic field to collapse. As a result, current is still flowing in the actuator coil, and the actuator rotor prevents the throttle valve connected thereto from moving in the closing direction before the next ON time of the control signal. That is, the system does not have time to react before the next positive pulse is received. As a result, the throttle valve and the actuator tend to stay in the same position in which they previously were positioned, until the actuator force becomes sufficiently large to move the throttle valve. This is undesirable, however, because a significant speed change will then occur instead of a small speed change, which will be clearly audible to the operator. The resulting speed may actually be outside of the specified speed band. In addition, this prior art system also causes undesirable speed hunting or speed oscillations.
One way to solve this problem would be to reduce the friction in the actuator by, for example, using ball bearings. This solution is unsatisfactory, however, because it substantially increases the cost of the actuator.
A second possible solution would be to reduce the inductance of the actuator. However, the actuator current must be correspondingly increased to yield the same actuator force. This solution is undesirable, however, because the use of higher current increases the heat output of the actuator coil, and requires higher temperature wiring between the control unit and the actuator.